Hydrogen is an extremely clean energy source for use in fuel cells and internal combustion engines. However, widespread use of hydrogen as a fuel will require innovations in hydrogen storage and hydrogen sensing. Reliable, cheap, compact, and safe hydrogen sensors are needed both for measuring the hydrogen concentration in flowing gas streams and for monitoring ambient air for leaked hydrogen. It is essential that “alarm” sensors detect hydrogen at a concentration well below the lower explosion limit in air of 4%.
The vast majority of hydrogen sensors use a palladium element to selectively absorb hydrogen. Such sensors operate by detecting a change in the properties of the palladium/hydrogen solution relative to those of pure palladium. The properties detected include mass, volume, electrical resistivity, optical constants, and the work function. Conventional palladium-based hydrogen sensors, however, have two main disadvantages: First, the response time for these devices, which tends to range from several minutes to 0.5 s, is too slow to permit useful, real-time monitoring of flowing gas streams. Second, palladium is poisoned by exposure to reactive species, such as hydrocarbons, O2, H2O, and CO, that chemisorb on the palladium surface and block adsorption sites needed for hydrogen. These species are exactly the sorts of contaminants that are likely to be present in the gaseous feed stream supplying a fuel cell or an internal combustion engine. Exposure of a palladium-based hydrogen sensor to these gases causes the response time for the sensor to increase, and can necessitate recalibration of the sensor for hydrogen.
Today, most hydrogen gas sensors are macroscopic palladium resistor-based sensors. Exposure to hydrogen gas causes an increase in the resistance in these devices by a factor of up to 1.8 at 25° C. The resistance increase is caused by the increased resistivity of palladium hydride relative to pure palladium. Although useful, these sensors not only suffer from the disadvantages noted above, they tend to require heating to operate efficiently, which tends to result in higher power consumption.
In view of such devices, it is desirable to provide a device sensitive to hydrogen, particularly a hydrogen gas sensor that is very easy to fabricate and responds very quickly to the presence of hydrogen gas.